Silicon photomultipliers (“SiPM”) are photo-sensing devices including a two-dimensionally arranged avalanche photodiode (“APD”) array. If a reverse-bias voltage higher than the APD breakdown voltage is applied to an APD, the APD is operated in Geiger mode. The gain of the APD in Geiger mode is very high, on the order of 105 to 106. This enables the APD to measure subtle light of a single photon. Since a high reverse-bias voltage needs to be applied to a SiPM to operate, typically the thickness of a depletion layer of each APD is 2 μm to 3 μm, and the reverse-bias voltage is 100V or less. The spectral sensitivity characteristic of a SiPM considerably depends on the absorption characteristic of silicon, and has a peak between 400 nm to 600 nm. The SiPMs are hardly sensitive to photons in the near-infrared wavelength band, which is 800 nm or more.
Some photo-sensing devices including silicon are known to have a very thick depletion layer, several tens μm, to become sensitive to photons in the near-infrared wavelength band. In this case, however, the drive voltage becomes very high, several hundreds V. Therefore, miniaturization APD arrays like those of SiPMs have not been achieved yet.
Other photo-sensing devices are known, in which the back side of a silicon substrate is made to have an irregular scattering surface by means of laser processing technology, so that unabsorbed light is reflected on the scattering surface. However, it is difficult to form a scattering and reflecting surface that is suitable for light in the near-infrared wavelength band. Furthermore, these photo-sensing devices require dedicated laser processing apparatus and method, which may increase the costs. In addition, mechanically processing a silicon layer of a diode included in a photo-sensing device may be equivalent to forming a detective layer, which may cause a problem in electric characteristics of the photo-sensing device such as stability, yield, and reproducibility.